1. Field of the Invention
The present invention relates to a method of fabricating a flash memory device and, more particularly, to a method of fabricating a flash memory device that can reduce incidences of an inter-bit line interference phenomenon by minimizing a phenomenon in which charges are trapped at a dielectric layer over an isolation structure.
2. Brief Description of Related Technologies
A “data storage” operation of a flash memory device is generally performed based on the principle that charges are stored in a floating gate according to a voltage applied to a control gate. A tunnel insulating layer is formed between the floating gate and a semiconductor substrate. A dielectric layer is formed between the floating gate and the control gate. The tunnel insulating layer functions to prevent charges, stored in the floating gate, from being drained. The dielectric layer functions to prohibit the transfer of charges between the floating gate and the control gate. A “program storage” operation of the flash memory device, based on the above principle, is described below.
In a memory cell string, the control gate is used as a word line. The word line is commonly connected to a plurality of memory cells constituting a plurality of memory cell strings. A voltage is transferred through the word line. If a voltage is applied to the word line, electrons within the control gate are rearranged, and a channel is formed in an active region under the tunnel insulating layer. This results in a tunneling phenomenon wherein electrons pass through the boundary of the tunnel insulating layer so that the electrons are stored in the floating gate.
The floating gate is separated by an isolation structure per cell basis. The dielectric layer and the word line are formed over the isolation structure. In this structure, if a high voltage is applied between the word line and the active region for the program operation, not only F-N (Fowle-Nordheim) tunneling occurs between the floating gate and the active region, but also unwanted current is generated between the word line and the active region. This results in a charge trap phenomenon wherein charges are trapped at layers constituting the dielectric layer, especially, the nitride layer. This causes to increase a change of the threshold voltage and degrade reliability of the device.
This phenomenon becomes more profound as the level of integration of devices increases. Accordingly, there is an urgent need for a solution for this problem.